C语言实现RSA算法数字签名不需要OpenSSL库

C语言实现RSA算法数字签名不需要OpenSSL库的方法是使用mbedtls库，这是一款轻量级的加密库。下面是一个使用mbedtls库实现RSA数字签名和验签的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "mbedtls/md5.h"
#include "mbedtls/rsa.h"

// 生成RSA密钥对
int generateRSAKey(mbedtls_rsa_context *rsa)
{
    int ret = mbedtls_rsa_gen_key(rsa, mbedtls_ctr_drbg_random, NULL, 2048, 65537);
    if (ret != 0)
    {
        printf("mbedtls_rsa_gen_key failed: %d\n", ret);
        return -1;
    }

    return 0;
}

// RSA数字签名
int rsaSign(const char* data, size_t dataLen, mbedtls_rsa_context *rsa, unsigned char* sig, size_t* sigLen)
{
    unsigned char md[MBEDTLS_MD_MAX_SIZE];
    mbedtls_md_context_t mdContext;
    mbedtls_md_init(&mdContext);
    const mbedtls_md_info_t* mdInfo = mbedtls_md_info_from_type(MBEDTLS_MD_MD5);
    mbedtls_md_setup(&mdContext, mdInfo, 0);
    mbedtls_md_starts(&mdContext);
    mbedtls_md_update(&mdContext, (const unsigned char*)data, dataLen);
    mbedtls_md_finish(&mdContext, md);
    mbedtls_md_free(&mdContext);

    int ret = mbedtls_rsa_pkcs1_sign(rsa, mbedtls_ctr_drbg_random, NULL, MBEDTLS_RSA_PRIVATE, MBEDTLS_MD_MD5, (unsigned int)(mdInfo->size), md, sig);
    if (ret != 0)
    {
        printf("mbedtls_rsa_pkcs1_sign failed: %d\n", ret);
        return -1;
    }

    *sigLen = mbedtls_rsa_get_len(rsa);

    return 0;
}

// RSA验签
int rsaVerify(const char* data, size_t dataLen, mbedtls_rsa_context *rsa, unsigned char* sig, size_t sigLen)
{
    unsigned char md[MBEDTLS_MD_MAX_SIZE];
    mbedtls_md_context_t mdContext;
    mbedtls_md_init(&mdContext);
    const mbedtls_md_info_t* mdInfo = mbedtls_md_info_from_type(MBEDTLS_MD_MD5);
    mbedtls_md_setup(&mdContext, mdInfo, 0);
    mbedtls_md_starts(&mdContext);
    mbedtls_md_update(&mdContext, (const unsigned char*)data, dataLen);
    mbedtls_md_finish(&mdContext, md);
    mbedtls_md_free(&mdContext);

    int ret = mbedtls_rsa_pkcs1_verify(rsa, mbedtls_ctr_drbg_random, NULL, MBEDTLS_RSA_PUBLIC, MBEDTLS_MD_MD5, (unsigned int)(mdInfo->size), md, sig);
    if (ret != 0)
    {
        printf("mbedtls_rsa_pkcs1_verify failed: %d\n", ret);
        return -1;
    }

    return 0;
}

int main()
{
    mbedtls_rsa_context rsa;
    mbedtls_rsa_init(&rsa, MBEDTLS_RSA_PKCS_V15, 0);

    // 生成RSA密钥对
    int ret = generateRSAKey(&rsa);
    if (ret != 0)
    {
        return -1;
    }

    // 要签名的数据
    char data[] = "Hello World!";

    // RSA数字签名
    unsigned char sig[MBEDTLS_MPI_MAX_SIZE];
    size_t sigLen = 0;
    ret = rsaSign(data, sizeof(data), &rsa, sig, &sigLen);
    if (ret != 0)
    {
        return -1;
    }

    // RSA验签
    ret = rsaVerify(data, sizeof(data), &rsa, sig, sigLen);
    if (ret != 0)
    {
        return -1;
    }

    printf("RSA verify success!\n");

    mbedtls_rsa_free(&rsa);

    return 0;
}

需要注意的是，本示例使用了mbedtls库来实现RSA签名和验签，使用前需要先安装mbedtls库并链接到项目中。